An automatic transmission mounted on a vehicle generally includes a plurality of planetary gear sets (planetary gear mechanisms) and a plurality of hydraulic pressure type frictional engagement elements such as clutches or brakes and is configured so that it switches a power transmission path going through each planetary gear set by selectively engaging these frictional engagement elements by hydraulic control and actualizes a plurality of forward gear shift stages and usually one stage of reverse gear shift. However, in recent years, it has become more desirable to provide multiple stages for the forward gear shift stage to improve engine fuel efficiency or shift performance. For example, one automatic transmission that has been considered includes three planetary gear sets and six frictional engagement elements and actualizes eight forward stages by engaging two of these frictional engagement elements.
However, according to this configuration, four frictional engagement elements in an unengaged state exist at each gear shift stage. As a result, the loss of driving force of the transmission as a whole is increased due to the sliding resistance among the friction plates of these frictional engagement elements or viscosity resistance of lubricant among the friction plates, possibly negating the improvements in fuel efficiency that are achieved by providing multiple stages for the forward gear shift stage.
On the other hand, Patent Document 1 discloses an automatic transmission that includes four planetary gear sets and five frictional engagement elements and actualizes forward eight stages by selectively engaging three of these frictional engagement elements. According to this Document, the number of the frictional engagement elements in the unengaged state at each gear shift stage becomes two, thereby suppressing losses in driving force as described above.